Rolling elements for bearings (rollers, needles, balls, races, etc.) used in the fields of various industrial machines and automobiles, etc. undergo high repeated stress in the radial direction (direction perpendicular to the axis of the rolling element). Accordingly, the rolling elements for bearings are required to have excellent rolling contact fatigue properties. The demands for rolling contact fatigue properties have become more stringent year after year in response to the trend of increasing the performance and reducing the weight in industrial machines. Bearing steel material is required to have better rolling contact fatigue properties in order to further improve the durability of bearing parts.
It was considered heretofore that the rolling contact fatigue properties intensely correlate with the number density of oxide inclusions formed in the steel, mainly, hard oxide inclusions such as Al2O3 formed when an Al deoxidized steel is used and the rolling contact fatigue properties are improved by reducing the number density of the hard oxide inclusions. Accordingly, it was attempted to improve the rolling contact fatigue properties by decreasing the oxygen content in the steel in the steel making process.
In recent years, however, a study has been progressed on the relation between the rolling contact fatigue properties and the non-metallic inclusions typically represented by oxide inclusions and it has been found that the number density of the oxide inclusions and the rolling contact fatigue properties are not always in a correlationship. That is, it has been revealed that the rolling contact fatigue properties are in a close correlation with the size of the non-metallic inclusions, for example, the square root of the area of the non-metallic inclusions and, for improving the rolling contact fatigue properties, it is more effective to decrease the size of the non-metallic inclusions than to reduce the number density of the non-metallic inclusions.
Then, instead of using the Al deoxidized steel as usual, there has been proposed a method of improving the rolling contact fatigue properties by suppressing the Al content in the steel as much as possible and forming Si deoxidized steel to control the composition of formed oxides to a composition mainly consisting of SiO2, CaO, etc. instead of the composition mainly consisting of Al2O3, thereby elongating and segmenting the non-metallic inclusions in the rolling step to decrease the size of the non-metallic inclusions.
For example, the Patent Literature 1 proposes a bearing steel material in which an average composition of oxides comprises, in mass %, 10 to 60% of CaO, 20% or less of Al2O3, 50% or less of MnO, 15% or less of MgO, and the balance of SiO2 and impurities, in which the value for the arithmetic mean of the maximum thickness of the oxides and the value for the arithmetic mean of the maximum thickness of sulfides present in an area of 100 mm2 at 10 locations of the vertical cross section in the longitudinal direction of the steel material are 8.5 μm or less respectively. As a method of manufacturing such a bearing steel material, there has been disclosed a method of elongating and segmenting not only oxides but also sulfides by strictly controlling the main constituent components of the slug in the process of secondary refinement and refining the oxides, and reducing the amount of S and appropriately controlling the rolling down conditions such as roll down ratio and the working temperature for sulfides which are difficult to be elongated or segmented by usual rolling down.
Further, the Patent Literature 2 discloses an Si deoxidized steel material at high cleanliness containing a predetermined amount of ZrO2 as an oxide component not known so far in the oxide inclusions described in the Patent Literature 1. The Patent Literature 2 describes that “ZrO2 used in a small amount contributes to retainment of an amorphous phase of the oxide inclusions and has an effect of suppressing the formation of other crystal phases when a crystal phase containing ZrO2 is formed. Accordingly, oxide inclusions remaining in the hot rolling and cold forging rolling step are segmented and refined more reliably than usual and the formation of coarse inclusions is suppressed”.
On the other hand, the technique taking notice on the interfacial peeling between the steel as the matrix and the non-metallic inclusions includes Patent Literature 3. The Patent Literature 3 proposes a technique of improving the rolling contact fatigue properties by controlling such that the difference of Young's modulus of steel and Young's modulus of the inclusions is decreased. The Patent Literature 3 has been accomplished based on the following finding. “The amount of Al in the steel manufactured by an existent Al deoxidizing step is about 0.015 to 0.025% and, as a result, since the inclusions containing much Al has a Young's modulus extremely higher compared with that of the steel (matrix), voids tend to be formed at the interface between the matrix and the inclusions and by way of the formation of the void, tensile strength exerts on the periphery of the void to readily occur cracks. Further, when soft non-metallic inclusions similar to voids are present in the steel, cracks are readily formed at the periphery thereof by the effect of the tensile stress”. It discloses a steel in which Al is less than 0.010% and Young's modulus ratio E2/E1 is controlled to a range of 0.3<E2/E1<1.6 assuming E1 as the Young's modulus of the steel and E2 as an average Young's modulus of inclusions at a size of 15 μm or more (maximum length in the vertical direction×the maximum length in the lateral direction)1/2 present in 3000 mm2 of a microscopic examination area in the steel when the size of the inclusions is defined as (maximum length in the vertical direction×the maximum length in the lateral direction)1/2.
That is, in the Patent Literature 3, Young's modulus ratio E2/E1 is controlled in a range of 0.3<E2/E1<1.6 with the recognition that the tensile strength exerts on the interface between the steel of the matrix and the non-metallic inclusions for the soft non-metallic inclusions at E2/E1 of 0.3 or less which is similar to voids or non-metallic inclusions at E2/E1 of 1.6 or more that tends to form voids to worsen the rolling contact fatigue properties.